Fixes for touch

Xaver Hugl requested to merge zamundaaa/kwin:work-around-touch-bug into master

Cancel events tended to make the touch focus get stuck because we did not count them as ending a touch sequence, this fixes that. I also replaced libinput_event_touch_get_slot with libinput_event_touch_get_seat_slot which works for multiple touch devices (I didn't check if the current code would fail with multiple touch screens but best case it fixes problems, worst case it changes IDs we don't care about)

Edited by Xaver Hugl

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